Devices operating in mobile communication systems, such as cellular telephones and other wireless devices, are configured to communicate over multiple wireless networks. Therefore, different signal paths of a mobile communication device may need to be connected to a common port, such as a common antenna node. For example, a mobile communication device includes a receiver and a transmitter that are typically connected to a common antenna through the common antenna node, for sending and receiving signals over the wireless network. The signal paths must be isolated from one another, and therefore include radio frequency (RF) filter circuits formed by one or more band pass filters that have passbands corresponding to the RF frequency bands of the respective signal paths.
For example, a duplexer is formed by two filter circuits and accommodates two signal paths (e.g., a receive path from a common antenna to a receiver and a transmit path from a transmitter to the common antenna). Each of the filter circuits is a band pass filter. Accordingly, the receiver is able to receive signals through a receive frequency passband, and the transmitter is able to send transmit signals through a different transmit frequency passband, while filtering out the other frequencies. More generally, a multiplexer has multiple (two or more) filter circuits and signal paths connected to a common port with corresponding filter circuits, each formed by one or multiple band pass filters.
The receive and transmit signals may be RF signals corresponding to various predetermined wireless communication standards, such as such as universal mobile telecommunications system (UMTS), global system for mobile communication (GSM), wideband code division multiple access (WCDMA), Long-Term Evolution (LTE) and LTE-Advanced, for example. The communication standards identify separate bands for transmitting and receiving signals. For example, LTE is allocated various 3GPP bands, including bands 1, 3, 7 and 41. Band 1 provides a transmit (uplink) frequency band of 1920 MHz-1980 MHz and a receive (downlink) frequency band of 2110 MHz-2170 MHz, band 7 provides a transmit frequency band of 2500 MHz-2700 MHz and a receive frequency band of 2620 MHz-2690 MHz, and band 41 provides a transmit/receive frequency band of 2496 GHz-2690 GHz. Accordingly, a multiplexer operating in compliance with a 3GPP standard would include filters having passbands within the corresponding transmit and receive frequency bands.
Bands supported with simultaneous operation may be referred to as carrier aggregation. LTE-Advanced, for example, offers higher data rates over the initial releases of LTE. To achieve these higher data rates, LTE-Advanced increases the transmission bandwidths over those that can be supported by a single carrier through carrier aggregation. Generally, carrier aggregation utilizes more than one carrier and hence increases the overall transmission bandwidth.
As the number of possible combinations of bands supported with simultaneous operation grows, it becomes more difficult to support all possible combinations with fixed multiplexers. To do so would require using multiple copies of some filter circuits, each multiplexed with some required combination of other aggregated bands. For example, one location (e.g., North America) and/or service provider may require band 41 to be multiplexed with band 25, while another location (e.g., China) and/or service provider may require band 41 to be multiplexed with bands 1 and 3. Since band 25 collides with band 3, a single multiplexer with all these bands (i.e., bands 1, 3, 25 and 41) is not possible. So, for a conventional mobile device to be able to accommodate both arrangements, there would have to be two different multiplexers with two copies of the band 41 filter included in the mobile device. This adds cost and increases area/size of the mobile device, both of which are undesirable consequences of carrier aggregation.
In RF communications, use of a common antenna requires matching a common antenna node (or terminal) to respective nodes (or terminal) of the multiple signal paths in their respective passbands to optimize signal transfer. This matching requirement drives the need for a combined matching network at the common antenna node that connects the filters associated with the different frequency bands for carrier aggregation.